The present invention relates to an automatic focusing apparatus for a video camera.
Conventional automatic focusing apparatus are classified roughly into two types.
Automatic focusing apparatus of a first type modulate the intensity of a beam of infrared rays at a predetermined frequency, project the modulated beam of infrared rays on an object, and detect a beam of infrared rays reflected by the object to determine the state of focus. Such automatic focusing apparatus are disclosed in U.S. Pat. No. 4,251,145 and Japanese Pat. Provisional Publication No. 57-20708. Such a known automatic focusing apparatus modulates the intensity of a beam of infrared rays at a predetermined frequency, projects the modulated beam of infrared rays on an object, converts a beam of infrared rays reflected by the object into an electric signal by a light receiving element, detects the electric signal by a synchronous detector, integrates the detected electric signal once every predetermined time interval to obtain a focus signal, and compares the focus signal with a reference voltage to decide the state of focus. In such an automatic focusing system, the magnitude of the light receiving element is very small and, sometimes, the offset voltages of the synchronous detector and the integrator are greater than the output of the light receiving element when the subject distance is great. In such a case, an erroneous decision regarding the state of focus results.
The conventional automatic focusing apparatus includes an offset voltage regulating means to reduce the offset voltage of the synchronous detector or the integrator to zero when no light is falling on the light receiving element. However, the offset voltages of the synchronous detector and the integrator vary depending on the ambient temperature and hence the associated circuits must be designed strictly so as to eliminate the influence of temperature variation on the automatic focusing operation.
An automatic focusing apparatus of the other type brings an object into focus by regulating the position of the taking lens so that the high-frequency component of a video signal provided by the video camera is increased to a maximum. Japanese Pat. Provisional Publication (Kokai) No. 58-48576 discloses such an automatic focusing apparatus. In such an automatic focusing system, the high-frequency component is subjected to full-wave detection and the detected signal is integrated once every fixed period of time to obtain a focus signal. In the automatic focusing apparatus of this type continuous regulation to reduce the offset voltage of the integrating circuit to zero is necessary.
Inventions relating to the present invention are disclosed in U.S. Pat. Appln. No. 858,792 (Title: "Automatic Focusing Apparatus", Filing date: May, 1986, Inventors: Hironobu Sato, Takesuke Maruyama, Takashi Azumi), U.S. Pat. Nos. 4,611,244, 4,320,417 and 4,531,158, and U.S. Pat. No. 4,740,676 (Title: Focusing Apparatus", Filing date: Sept. 19, 1987, Inventors: Kenji Sano, Hironobu Sato, Takesuke Maruyama, Takanori Hisada, Takashi Azumi).